1. Field of the Invention
The present invention relates to a solid electrolytic capacitor using a conductive polymer as a solid electrolyte, and to a method for manufacturing the solid electrolytic capacitor.
2. Description of Related Art
In the wake of digitalization and the trend towards smaller, higher-speed electronic devices, there is a steadily growing demand for small, large-capacitance capacitors having low impedance at high frequencies, also in the field of solid electrolytic capacitors.
Conventional capacitors used in high frequencies ranging from 100 kHz to several tens of MHz include mica capacitors and laminated ceramic capacitors. Such capacitors, however, are large, and can yield substantial capacitances only with difficulty. Meanwhile, electrolytic capacitors such as aluminum electrolytic capacitors or tantalum solid electrolytic capacitors are used as large-capacitance capacitors. However, the electrolytes used in such electrolytic capacitors (an electrolyte solution in aluminum electrolytic capacitors and manganese dioxide in tantalum electrolytic capacitors) have low conductivity. Therefore, it is difficult to obtain capacitors having low enough impedance at high frequencies.
To overcome the above drawbacks, Japanese Examined Patent Application Publication No. H04-56445 discloses an aluminum solid electrolytic capacitor and a tantalum solid electrolytic capacitor that uses, as an electrolyte, a high-conductivity conductive polymer such as polypyrrole or polythiophene. The electrolyte in a solid electrolytic capacitor using such a conductive polymer has high conductivity, thanks to which the solid electrolytic capacitor features low enough impedance at high frequencies.
In such a solid electrolytic capacitor using a conductive polymer on a solid electrolyte layer, a conductive polymer layer peels off a dielectric oxide film, whereby contact resistance increases, when adherence between the dielectric oxide film layer and the conductive polymer layer is poor. This is problematic in that, as a result, good equivalent series resistance (ESR) cannot be obtained.
To solve the problem of adherence between the dielectric oxide film layer and the conductive polymer layer, Japanese Patent No. 3241636 discloses a method of forming a conductive polymer, in which an acrylic resin and/or a cellulose resin is added to a conductive polymer monomer, followed then by a polymerization reaction.
Manufacturing a solid electrolytic capacitor by adding an acrylic resin and/or a cellulose resin to a conductive monomer, in accordance with a conventional technology, has the effect of increasing adherence between the dielectric oxide film layer and the conductive polymer layer vis-a-vis the case where no additive is added. However, it has been found that part of the conductive polymer still peels off and that contact resistance increases.